Fix server initialization race condition with proper event signaling

src/py_dvt_ate/app/dashboard/app.py

@@ -61,25 +61,34 @@ def start_embedded_server() -> tuple[SimulationServer, threading.Thread]:
         )
     )
 
+    server_ready = threading.Event()
+
     def run_server() -> None:
         """Run the async server in a new event loop."""
         loop = asyncio.new_event_loop()
         asyncio.set_event_loop(loop)
         try:
             loop.run_until_complete(server.start())
+            # Signal that server is ready
+            server_ready.set()
             # Keep the event loop running
             loop.run_forever()
         except Exception as e:
             st.error(f"Server error: {e}")
+            server_ready.set()  # Signal even on error
         finally:
-            loop.run_until_complete(server.stop())
+            try:
+                loop.run_until_complete(server.stop())
+            except Exception:
+                pass
             loop.close()
 
     thread = threading.Thread(target=run_server, daemon=True)
     thread.start()
 
-    # Wait a moment for server to start
+    # Wait for server to be fully started (up to 5 seconds)
-    time.sleep(0.5)
+    if not server_ready.wait(timeout=5.0):
+        st.error("Server failed to start within timeout")
 
     return server, thread
 
@@ -87,13 +96,22 @@ def start_embedded_server() -> tuple[SimulationServer, threading.Thread]:
 def init_session_state() -> None:
     """Initialise Streamlit session state."""
     if "server" not in st.session_state:
-        st.session_state.server, st.session_state.server_thread = start_embedded_server()
+        with st.spinner("Starting simulation server..."):
+            st.session_state.server, st.session_state.server_thread = start_embedded_server()
+
+        # Verify server started correctly
+        if st.session_state.server.physics_engine is None:
+            st.error("Failed to start simulation server. Please refresh the page.")
+            st.stop()
 
         # Register cleanup
         def cleanup() -> None:
             if hasattr(st.session_state, "server") and st.session_state.server is not None:
                 loop = asyncio.new_event_loop()
-                loop.run_until_complete(st.session_state.server.stop())
+                try:
+                    loop.run_until_complete(st.session_state.server.stop())
+                except Exception:
+                    pass
                 loop.close()
         atexit.register(cleanup)
 
@@ -308,7 +326,24 @@ def display_controls() -> None:
 @st.fragment(run_every=0.1)
 def simulation_display() -> None:
     """Fragment that displays and updates simulation state."""
+    # Check if server is initialized
+    if "server" not in st.session_state:
+        st.warning("⏳ Initializing simulation server...")
+        return
+
     server: SimulationServer = st.session_state.server
+
+    # Get current state from physics engine (for visualization)
+    engine: PhysicsEngine | None = server.physics_engine
+    if engine is None:
+        st.error("❌ Physics engine not available. The server may not have started correctly. Try refreshing the page.")
+        return
+
+    # Check if server is running
+    if not server.is_running:
+        st.warning("⚠️ Server is not running. Try refreshing the page.")
+        return
+
     instruments: InstrumentSet = st.session_state.instruments
     history: SimulationHistory = st.session_state.history
 
@@ -319,12 +354,6 @@ def simulation_display() -> None:
     if st.session_state.running:
         step_simulation()
 
-    # Get current state from physics engine (for visualization)
-    engine: PhysicsEngine | None = server.physics_engine
-    if engine is None:
-        st.error("Physics engine not available")
-        return
-
     thermal = engine.get_thermal_state()
     electrical = engine.get_electrical_state()